1. Field of the Invention
The invention relates to a device for operating a fluorescent discharge lamp of the outer electrode type which is used for document scanning illumination of an information processing device, such as a fax machine, a copier, an image reader and the like, and for a back light device of a liquid crystal display and for similar purposes.
2. Description of the Related Art
A fluorescent discharge lamp of the outer electrode type is known as a fluorescent lamp which is used for document scanning illumination of an office automation device and for back light of a liquid crystal display of an office automation device and the like.
In the above described fluorescent discharge lamp of the outer electrode type, on the outside of a glass tube there is a pair of line-like or strip-like outer electrodes to which an uninterrupted high frequency voltage or a pulse-like high frequency voltage is applied to operate the lamp.
FIG. 12 is a schematic of an arrangement of the above described fluorescent discharge lamp of the outer electrode type. FIG. 12(a) shows a cross section of the fluorescent discharge lamp of the outer electrode type which corresponds to the direction perpendicular to the direction of the tube axis. FIG. 12(b) is a side view hereof.
As becomes apparent from FIG. 12, fluorescent discharge lamp of the outer electrode type 1 consists of discharge vessel 3 which consists of a dielectric such as glass or the like, a pair of strip-like or line-like electrodes 2, 2', and layers of fluorescent material 4 which are formed on the inside of discharge vessel 3. Electrodes 2, 2' are located on the side of discharge vessel 3 in the direction of the tube axis roughly over the entire length and consist of aluminum of the like.
Conventionally, above described fluorescent discharge lamp of the outer electrode type 1 has been operated by applying a high frequency voltage to the outer electrodes as in a cold cathode fluorescent discharge lamp of the inner electrode type. That is, high frequency main circuit 5 as shown in FIG. 13 is connected to the pair of electrodes 2, 2' of fluorescent discharge lamp of the outer electrode type 1. For example, a waveform high frequency AC voltage according to FIG. 14 is applied to electrodes 2, 2'. In this way a high frequency voltage is applied in the discharge space within discharge vessel 3 which is present between outer electrodes 2, 2' via the side discharge vessel 3, forming a discharge.
This approach is disclosed in the patent disclosure document of Japanese patent application HEI 3-225745 (U.S. Pat. No. 5,117,160).
In the above described conventional operation method there is a process with which the illuminance of the fluorescent discharge lamp of the outer electrode type is increased even more. In this process the gas pressure of the rare gas which is filled within discharge vessel 3 is increased.
In the above described case of increasing the filling pressure however the current required for discharge is not sufficient if the voltage applied to the fluorescent discharge lamp of the outer electrode type is not increased. In this case a stable discharge cannot be obtained.
In the following the shape of the discharge within discharge vessel 3 is detailed in the case of increasing the encapsulation pressure:
In this discharge, over the entire region of the lamp a plurality of locations at which emission is concentrated is formed. This plurality of local sites at which the emission is concentrated furthermore changes their positions over time. The emission state in discharge vessel 3 is observed in the form of a strip. FIG. 15 schematically shows this strip-like discharge phenomenon.
Fluorescent discharge lamp of the outer electrode type 1 shown in FIG. 12 can be called a capacitor which consists of outer electrodes 2, 2' and discharge vessel 3 as the dielectric. The lamp current which is supplied to this fluorescent discharge lamp of the outer electrode type 1 is determined by the magnitude of electrostatic capacity which is formed between outer electrodes 2, 2' and discharge vessel 3 which consists of the dielectric. Current must be supplied to increase the illuminance of fluorescent discharge lamp of the outer electrode type 1 even more. This means that the voltage applied to outer electrodes 2, 2' and the high frequency of the voltage must be increased.
When the applied voltage was increased however the disadvantages of the danger of formation of creeping discharge on the surface of the dielectric between outer electrodes 2, 2' and the danger of formation of an insulation breakdown in the circuit arose.